1. Field of the Invention
The present invention relates to drive units that control the driving of a load (such as a light-emitting diode). The present invention also relates to smoothing circuits that smooth an alternating-current signal by using an output capacitor connected in parallel with the load, and to DC/DC converters that convert an input voltage into a desired output voltage and feed it to the load.
2. Description of Related Art
Currently, as a backlight of an LCD (liquid crystal display) panel (for example, a monitor of a car navigation system), cold cathode fluorescent lamps (CCFL) are mainly used. In recent years, however, white LEDs (light-emitting diodes) have been put to practical use for their advantages such as light weight, thinness, resistance to vibration and impact, wide brightness adjustment range, power-saving, longer life, low drive voltage, and being free from Hg, and various technologies related to LED drive units (so-called LED drivers) that control the driving of such white LEDs have been disclosed and proposed (see, for example, JP-A-2007-13183 (hereinafter “Patent Document 1”)).
As LEDs have come to be increasingly used as a backlight, the LED drivers that drive the LEDs are required to have a longer battery life or to reduce power consumption by reducing the brightness of a screen, and accordingly LED drive units are required to have the capability of controlling the LED brightness at lower brightness levels with a high degree of accuracy.
Like the above-described LED drive units, drive units that drive loads other than LEDs are required to have the capability of controlling the load at lower driving levels with a high degree of accuracy.
In addition, with respect to a DC/DC converter that converts an input voltage into a desired output voltage and feeds it to the load, the following problem arises. If voltage variation appears in an input bypass capacitor which is an input smoother of the DC/DC converter or an output capacitor which is an output smoother of the DC/DC converter, the device may expand and contract, causing a substrate to tremble and make a sound. This sound is easily produced especially when the input bypass capacitor or the output capacitor has a large capacitance, because this results in an increase in device size.